A nuclear isomer is a metastable state of an atomic nucleus caused by the excitation of one or more of its nucleons. Metastability is a general scientific concept which describes states of delicate equilibrium The nucleus of an Atom is the very dense region consisting of Nucleons ( Protons and Neutrons, at the center of an atom Excitation is an elevation in energy level above an arbitrary baseline energy state In Physics a nucleon is a collective name for two Baryons the Neutron and the Proton. A nuclear isomer occupies a higher energy state than the corresponding non-excited nucleus, called the ground state. In Quantum mechanics, a stationary state is an Eigenstate of a Hamiltonian, or in other words a state of definite energy The nuclear isomer will eventually release the extra energy and decay into the ground state.
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Metastable isomers
Metastable isomers can be produced through nuclear fusion or other nuclear reactions. In Physics and Nuclear chemistry, nuclear fusion is the process by which multiple- like charged atomic nuclei join together to form a heavier nucleus In Nuclear physics, a nuclear reaction is the process in which two nuclei or nuclear particles collide to produce products different from the initial particles A nucleus thus produced generally starts its existence in an excited state that de-excites through the emission of one or more gamma rays (or, equivalently, conversion electrons), usually in a time far shorter than a picosecond. Excitation is an elevation in energy level above an arbitrary baseline energy state Gamma rays (denoted as &gamma) are a form of Electromagnetic radiation or light emission of frequencies produced by sub-atomic particle interactions A conversion electron is a term used to describe an Electron which results from the Internal conversion process To help compare Orders of magnitude of different Times this page lists times between 10&minus12 seconds and 10&minus11 seconds (1 Pico However, sometimes it happens that the de-excitation does not proceed rapidly all the way to the nuclear ground state. In Quantum mechanics, a stationary state is an Eigenstate of a Hamiltonian, or in other words a state of definite energy This usually occurs because of the formation of an intermediate excited state with a spin far different from that of the ground state. In Quantum mechanics, spin is a fundamental property of atomic nuclei, Hadrons and Elementary particles For particles with non-zero spin Gamma-ray emission is far slower (is "hindered") if the spin of the post-emission state is very different from that of the emitting state, particularly if the excitation energy is low, than if the two states are of similar spin. The excited state in this situation is therefore a good candidate to be metastable, if there are no other states of intermediate spin with excitation energies less than that of the metastable state.
Metastable isomers of a particular isotope are usually designated with an "m" (or, in the case of isotopes with more than one isomer, m2, m3, and so on). Isotopes (Greek isos = "equal" tópos = "site place" are any of the different types of atoms ( Nuclides This designation is usually placed after the atomic symbol and number of the atom (e. g. , Co-58m), but is sometimes placed as a superscript before (e. g. , 58mCo). Increasing indices, m, m2, etc. correlate with increasing levels of excitation energy stored in each of the isomeric states (e. g. , Hf-177m2 or 177m2Hf).
A different kind of metastable nuclear state (isomer) is the fission isomer or shape isomer. Nuclear fission is the splitting of the nucleus of an atom into parts (lighter nuclei) often producing Free neutrons and other smaller nuclei which may Most actinide nuclei, in their ground states, are not spherical, but rather spheroidal — specifically, prolate, with an axis of symmetry longer than the other axes (similar to an American football or rugby ball, although with a less pronounced departure from spherical symmetry). History of the actinoid series From the earlier known chemical properties of actinium (89 up to uranium (92 indicating a relation to the Transition metals it was generally Equation A spheroid centered at the origin and rotated about the z axis is defined by the implicit equation \left(\frac{x}{a}\right^2+\left(\frac{y}{a}\right^2+\left(\frac{z}{b}\right^2 Reflection symmetry, line symmetry, mirror symmetry, mirror-image symmetry, or bilateral symmetry is Symmetry with respect American football, known in the United States and Canada simply as football, is a competitive Team sport known for mixing strategy with Rugby football (usually just " rugby " may refer to a number of sports through history descended from a common form of Football developed at Rugby School This article is about rotations in three-dimensional Euclidean space In some of these, quantum-mechanical states can exist in which the distribution of protons and neutrons is farther yet from spherical (in fact, about as non-spherical as an American football), so much so that de-excitation to the nuclear ground state is strongly hindered. The proton ( Greek πρῶτον / proton "first" is a Subatomic particle with an Electric charge of one positive This article is a discussion of neutrons in general For the specific case of a neutron found outside the nucleus see Free neutron. In general these states either de-excite to the ground state (albeit far more slowly than a "usual" excited state) or undergo spontaneous fission with half lives of the order of nanoseconds or microseconds — a very short time, but many orders of magnitude longer than the half life of a more usual nuclear excited state. Spontaneous fission (SF is a form of Radioactive decay characteristic of very heavy Isotopes and is theoretically possible for any atomic nucleus whose mass is greater Half-Life (computer-game page here It's already listed in the disambiguation page A nanosecond ( ns) is one billionth of a second See also times of other orders of magnitude. To help compare Orders of magnitude of different Times this page lists times between 10&minus6 seconds and 10&minus5 seconds (1 micro Fission isomers are usually denoted with a postscript or superscript "f" rather than "m," so that a fission isomer in e. g. plutonium 240 is denoted Pu-240f or 240fPu.
Nearly-stable isomers
Most nuclear isomers are very unstable, and radiate away the extra energy immediately (on the order of 10-12 seconds). As a result, the term is usually restricted to refer to isomers with half-lives of 10-9 seconds or more. Quantum mechanics predicts that certain atomic species will possess isomers with unusually long lifetimes even by this stricter standard, and so have interesting properties. Quantum mechanics is the study of mechanical systems whose dimensions are close to the Atomic scale such as Molecules Atoms Electrons By definition, there is no such thing as a "stable" isomer; however, some are so long-lived as to be nearly stable, and can be produced and observed in quantity.
The only nearly-stable nuclear isomer occurring in nature is Ta-180m, which is present in all tantalum samples at about 1 part in 8,300. Tantalum (ˈtæntələm (formerly tantalium /tænˈtæliəm/ is a Chemical element with the symbol Ta and Atomic number 73 Its half-life is at least 1015 years, markedly longer than the age of the universe. The age of the Universe is the time elapsed between the theory of the Big Bang and the present day This remarkable persistence results from the fact that the excitation energy of the isomeric state is low and both gamma de-excitation to the Ta-180 ground state (which is radioactive and not particularly long lived) and beta decay to hafnium or tungsten are suppressed owing to spin mismatches. In Nuclear physics, beta decay is a type of Radioactive decay in which a Beta particle (an Electron or a Positron) is emitted Hafnium (ˈhæfniəm is a Chemical element that has the symbol Hf and Atomic number 72 Tungsten (ˈtʌŋstən also known as wolfram (/ˈwʊlfrəm/ is a Chemical element that has the symbol W and Atomic number 74 The origin of this isomer is mysterious, though it is believed to have been formed in supernovas (as are most other heavy elements). A supernova (plural supernovae or supernovas) is a stellar Explosion. When it relaxes to its ground state, it releases a photon with an energy of 75 keV. In Physics, the photon is the Elementary particle responsible for electromagnetic phenomena It was first reported in 1988 by Collins[1] that Ta-180m can be forced to release its energy by weaker x-rays. After 11 years of controversy those claims were confirmed in 1999 by Belic and co-workers in the Stuttgart nuclear physics group. [2]
Another reasonably stable nuclear isomer (with a half-life of 31 years) is hafnium-178m2, which has the highest excitation energy of any comparably long-lived isomer. Hafnium (ˈhæfniəm is a Chemical element that has the symbol Hf and Atomic number 72 One gram of pure Hf-178-m2 contains approximately 1330 megajoules of energy, the equivalent of exploding about 317 kilograms (700 pounds) of TNT. For other uses of the words gram or gramme see Gram (disambiguation. Trinitrotoluene ( TNT) is a Chemical compound with the formula C6H2(NO23CH3 Further, in the natural decay of Hf-178-m2, the energy is released as gamma rays with a total energy of 2. 45 MeV. As with Ta-180m, there are disputed reports that Hf-178-m2 can be stimulated into releasing its energy, and as a result the substance is being studied as a possible source for gamma ray lasers. In Optics, stimulated emission is the process by which an electron perturbed by a Photon having the correct energy may drop to a lower Energy level resulting A laser is a device that emits Light ( Electromagnetic radiation) through a process called Stimulated emission. These reports also indicate that the energy is released very quickly, so that Hf-178-m2 can produce extremely high powers (on the order of exawatts). This page lists examples of the power in Watts produced by various different sources of energy Other isomers have also been investigated as possible media for gamma-ray stimulated emission. [3]
Applications
These hafnium and tantalum isomers have been considered in some quarters as weapons that could be used to circumvent the Nuclear Non-Proliferation Treaty, since they can be induced to emit very strong gamma radiation. DARPA has or has had a program to investigate this usage of both isomers. Hafnium (ˈhæfniəm is a Chemical element that has the symbol Hf and Atomic number 72 Tantalum (ˈtæntələm (formerly tantalium /tænˈtæliəm/ is a Chemical element with the symbol Ta and Atomic number 73 The Treaty on the Non-Proliferation of Nuclear Weapons, also Nuclear Non-Proliferation Treaty ( NPT or NNPT) is a Treaty to limit the spread In Physics, induced gamma emission ( IGE) refers to the process of fluorescent emission of Gamma rays from excited nuclei usually involving a specific The Defense Advanced Research Projects Agency (DARPA is an agency of the United States Department of Defense responsible for the development of new Technology However, given the difference in speed between a photon and a neutron, they can't be induced to chain react like a nuclear weapon, so there will probably never be such a weapon. Ta-180m is also one of the most expensive substances to procure in the world: It costs approximately $17 million per gram. In 1999, the entire world's supply of Ta-180m was only 6. 7 milligrams. [4]
Technetium isomers Tc-99m (with a half-life of 6. Technetium (tɛkˈniːʃɪəm is the lightest Chemical element with no Stable isotope. 01 hours) and Tc-95m (with a half-life of 61 days) are used in medical and industrial applications. Technetium (tɛkˈniːʃɪəm is the lightest Chemical element with no Stable isotope. Technetium (tɛkˈniːʃɪəm is the lightest Chemical element with no Stable isotope.
Decay processes
Isomers decay to lower energy states of the nuclide through two isomeric transitions:
- γ (gamma) emission (emission of a high-energy photon)
- internal conversion (the energy is used to ionize the atom)
See also
References
- ^ C.B. Collins et al., Phys. Rev. C, 37, p 2267-2269 (1988).
- ^ D. Belic et al., Phys. Rev. Lett., 83, p 5242 (1999).
- ^ UNH researchers search for stimulated gamma ray emission. Isomeric transition is a Radioactive decay process that occurs in an Atom where the nucleus is in an excited Meta state (e Internal conversion is a Radioactive decay process where an excited nucleus interacts with an Electron in one of the lower electron shells causing the In Physics, induced gamma emission ( IGE) refers to the process of fluorescent emission of Gamma rays from excited nuclei usually involving a specific UNH Nuclear Physics Group (April 1997). Retrieved on 2006-06-01. Year 2006 ( MMVI) was a Common year starting on Sunday of the Gregorian calendar. Events 193 - Roman Emperor Didius Julianus is Assassinated 987 - Hugh Capet is elected
- ^ Sunday Supplement Magazine Washington Post article, March 28, 2004
External links
- Research group which presented initial claims of hafnium nuclear isomer de-excitation control. - The Center for Quantum Electronics, The University of Texas at Dallas. The Washington Post is the largest and most circulated Newspaper in Washington D
- JASON Defense Advisory Group report on high energy nuclear materials mentioned in the Washington Post story above
- May 2004 article in Physics Today which reviews the Hf controversy in a balanced manner.
- Confidence for Hafnium Isomer Triggering in 2006. - The Center for Quantum Electronics, The University of Texas at Dallas. JASON is an independent group of scientists which advises the United States Government on matters of science and technology
- Reprints of articles about nuclear isomers in peer reviewed journals. - The Center for Quantum Electronics, The University of Texas at Dallas.
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